The_Catalyst_Review_November_2023 - 16

Experimental Abstracts
Nitrogen Reduction Reaction to Ammonia on Transition Metal Carbide Catalysts
Efforts are underway to replace the Haber-Bosch
process with a more environmentally friendly NH3
synthesis technique to save energy and reduce CO2
emissions. The electrocatalytic reduction of N2
under
ambient circumstances represents one such approach.
However, finding efficient N2
reduction reaction (NRR)
electrocatalysts has proven challenging due to the
high activation barrier associated with breaking the N2
triple bonds. Herein, the authors describe the activity of
transition metal carbide (TMC) surfaces in the (100) facets
of the rocksalt (RS) structure as prospective NRR catalysts.
To measure the overall performance of each TMC surface,
density functional theory (DFT) was used to simulate
reaction routes, estimate stability, assess kinetic barriers,
and compare adsorbate energies. Three promising
candidates were identified as a result of this study.
These workers began by selecting a structure and surface
orientation modeled with a 5-layer 2×2 unit cell containing
20 metal atoms and 20 carbon atoms (Figure 1). The
two bottom layers were fixed in place while the three
top layers, along with any adsorbates, were allowed to
relax. The reactivity of TMC surfaces was then tested by
modeling adsorption energies and reaction pathways
for each candidate: VC, CrC, YC, HfC, NbC, ScC, ZrC,
TiC, TaC, WC, and MoC. Their studies showed
that these TMCs are limited in their reactivity
due to endergonic nitrogen adsorption. ZrC was
the only TMC that could semi-favorably adsorb
nitrogen and produce ammonia. However, ZrC
is prone to protonate carbon sites over nitrogen
and is therefore not recommended as a selective
NRR catalyst in aqueous solution. However, it
does hold potential for use in non-aqueous
electrolytic systems.
Because these surfaces are prone to form
surface carbon vacancies at their respective
operating conditions, the authors felt that the
unfavorable nitrogen adsorption observed
for these TMCs could be circumvented in the
presence of a carbon vacancy. Indeed, when
such a vacancy was introduced, most TMC
surfaces favorably adsorbed nitrogen, but only
WC, NbC, and VC produced ammonia. A lack
of ammonia formation was observed for the
TMCs that featured highly exergonic nitrogen
adsorption and is likely due to the stability
gained by having nitrogen in the vacancy,
outweighing the tendency for protonation.
In the presence of a vacancy, VC, NbC, and WC
showed efficient nitrogen adsorption, selectivity
towards ammonia, and a low overpotential, as noted in Figure 2. However, NbC was disqualified due to the high activation energy
for the dissociative adsorption of N2 under ambient conditions. It may, however, be suitable under other temperature and pressure
conditions. For VC, poisoning of the vacancy site by adsorption of a proton was found to hinder the activity of the catalyst, causing
subsequent protonation of carbon sites over the adsorption of N2. WC proved to be the most promising candidate, displaying
good activity, stability, resistance to poisoning, low OP (- 0.35 V), and selectivity. Ellingsson V, Iqbal A, Skúlason E, et al. (2023).
ChemSusChem, doi.org/10.1002/cssc.202300947
to their respective name under each column.
16
The Catalyst Review
Figure 2. Comparison of binding energies of protons, hydroxyl, and oxygen
species in the vacancy of the TMCs, only the TMCs that favorably bind nitrogen
dissociatively are shown. The energies are plotted as the difference between the
adsorption energy of the relevant species, at the required OP for operation, and the
dissociative adsorption of N2
. The required OP for each material can be found next
Figure 1: A schematic representation of a 5-layer WC in the (100) facets of
the RS surface as used in the computational calculations. A top view of the
unit cell used can be found in the upper right-hand corner, denoted with a
black square.
November 2023

http://www.doi.org/10.1002/cssc.202300947

The_Catalyst_Review_November_2023

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